Preheat oven to 350°F (175°C). Grease and flour a 9x9 inch baking pan.

In a saucepan, melt butter over medium heat. Remove from heat and whisk in sugar, cocoa powder, and salt.

Whisk in vanilla extract and eggs, one at a time. Stir in flour until just combined.

In a small bowl, whisk together peanut butter, powdered sugar, and milk until smooth.

Pour brownie batter into prepared pan. Drop spoonfuls of peanut butter mixture over the batter. Swirl with a knife.

Bake for 25-30 minutes, or until a toothpick inserted into the center comes out with moist crumbs.

Let cool completely before cutting and serving.